1. Field of the Invention
This invention relates to electromagnetically driven shutters, and more particularly to electromagnetically driven shutters having diaphragm-cum-shutter blades of which the opening and closing operation is controlled by a coil arranged to travel forwards and backwards in magnetic fields.
2. Description of the Prior Art
In the past, the electromagnetically driven shutters have been constructed in such form as, for example, illustrated in FIGS. 1a and 1b, wherein the optical axis of the objective lens is used as a center A of rotation of a coil 7, and the wire of coil 7 is wound symmetrically with respect to this rotation center A. This has the advantages that, as the center of inertia of the coil 7 coincides with the center of rotation A thereof, any variation of the speed of forward and backward movement of the coil 7 depending on variation of the attitude of the shutter can be avoided, and the operation of winding turns of the coil 7 becomes easy. However, such form has the problem that the coil 7 is necessarily shaped to a large bulk and size.
Under this circumstance, when the coil 7 is given a constant electrical resistance, the number of turns of the coil 7 must be lessened with the result that the number of wire components which lie across the magnetic field formed by one magnet is reduced. Therefore it has been impossible to achieve a much-desired increase in the magnitude of electromagnetic force exerted on the coil 7 by one magnet. In more detail, letting F denote the electromagnetic force, B the strength of the magnetic field formed by the magnet, I the current flowing through the coil 7 and L the length of that part of the coil 7 which lies in the magnetic field, we have the electromagnetic force expressed as follows: EQU F=BIL (1)
It is evident from this equation that as the number of turns of the coil 7 decreases and the length of that part of the coil 7 which passes across each magnetic field shortens, the magnitude of electromagnetic force F is decreased.
In the conventional electromagnetically driven shutters, therefore, it has been required to employ a rather large number of precious magnet pieces (5a-5d, 6a-6d) than is usually expected as illustrated in FIGS. 1a and 1b (four pieces in FIG. 1a and eight pieces in FIG. 1b), thereby giving a disadvantage from the economical standpoint.